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Muon tracking in a LiquidO opaque scintillator detector

LiquidO Collaboration, J. Apilluelo, L. Asquith, E. F. Bannister, N. P. Barradas, C. L. Baylis, J. L. Beney, M. Berberan e Santos, X. de la Bernardie, T. J. C. Bezerra, M. Bongrand, C. Bourgeois, D. Breton, J. Busto, A. Cabrera, A. Cadiou, E. Calvo, M. de Carlos Generowicz, E. Chauveau, B. J. Cattermole, M. Chen, P. Chimenti, D. F. Cowen, S. Kr. Das, S. Dusini, A. Earle, M. Felizardo, C. Frigerio Martins, J. Galán, J. A. García, A. Gibson-Foster, C. Girard-Carillo, W. C. Griffith, J. J. Gómez-Cadenas, M. Guitière, F. Haddad, J. Hartnell, A. Holin, I. G. Irastorza, I. Jovanovic, A. Kling, L. Koch, P. Lasorak, J. F. Le Du, F. Lefevre, P. Loaiza, J. A. Lock, G. Luzón, J. Maalmi, J. P. Malhado, F. Mantovani, J. G. Marques, C. Marquet, M. Martínez, J. T. Moffat, D. Navas-Nicolás, H. Nunokawa, J. P. Ochoa-Ricoux, T. Palmeira, C. Palomares, D. Petyt, P. Pillot, A. Pin, J. C. C. Porter, M. S. Pravikoff, S. Richards, N. Rodrigues, M. Roche, R. Rosero, B. Roskovec, M. L. Sarsa, S. Schoppmann, A. Serafini, C. Shepherd-Themistocleous, W. Shorrock, M. Silva, L. Simard, S. R. Soleti, D. Stocco, V. Strati, J. S. Stutzmann, F. Suekane, N. Tuccori, A. Verdugo, B. Viaud, S. M. Wakely, A. Weber, G. Wendel, A. S. Wilhelm, A. W. R. Wong, M. Yeh, F. Yermia

TL;DR

This work demonstrates event-by-event muon tracking in a LiquidO opaque scintillator detector, using a 64-fibre Cube prototype read out by 128 SiPM channels. The wax-based opaque scintillator induces stochastic light confinement, achieving a per-row muon-position resolution of $450$ $μ$m—significantly better than the $730$ $μ$m observed with a transparent scintillator. The study details data acquisition, offline muon selection, and a ToT linearisation method that enables reliable light yield correction, and it compares the LiquidO performance to state-of-the-art muon-imaging systems, showing substantial potential for mm/sub-mm imaging in optimised detectors. These results establish LiquidO as a scalable, high-resolution alternative for muon imaging and related applications, with clear pathways for further improvement via material, geometry, and advanced reconstruction techniques.

Abstract

LiquidO is an innovative radiation detector concept. The core idea is to exploit stochastic light confinement in a highly scattering medium to self-segment the detector volume. In this paper, we demonstrate event-by-event muon tracking in a LiquidO opaque scintillator detector prototype. The detector consists of a 30 mm cubic scintillator volume instrumented with 64 wavelength-shifting fibres arranged in an 8$\times$8 grid with a 3.2 mm pitch and read out by silicon photomultipliers. A wax-based opaque scintillator with a scattering length of approximately 0.5 mm is used. The tracking performance of this LiquidO detector is characterised with cosmic-ray muons and the position resolution is demonstrated to be 450 $μ$m per row of fibres. These results highlight the potential of LiquidO opaque scintillator detectors to achieve fine spatial resolution, enabling precise particle tracking and imaging.

Muon tracking in a LiquidO opaque scintillator detector

TL;DR

This work demonstrates event-by-event muon tracking in a LiquidO opaque scintillator detector, using a 64-fibre Cube prototype read out by 128 SiPM channels. The wax-based opaque scintillator induces stochastic light confinement, achieving a per-row muon-position resolution of m—significantly better than the m observed with a transparent scintillator. The study details data acquisition, offline muon selection, and a ToT linearisation method that enables reliable light yield correction, and it compares the LiquidO performance to state-of-the-art muon-imaging systems, showing substantial potential for mm/sub-mm imaging in optimised detectors. These results establish LiquidO as a scalable, high-resolution alternative for muon imaging and related applications, with clear pathways for further improvement via material, geometry, and advanced reconstruction techniques.

Abstract

LiquidO is an innovative radiation detector concept. The core idea is to exploit stochastic light confinement in a highly scattering medium to self-segment the detector volume. In this paper, we demonstrate event-by-event muon tracking in a LiquidO opaque scintillator detector prototype. The detector consists of a 30 mm cubic scintillator volume instrumented with 64 wavelength-shifting fibres arranged in an 88 grid with a 3.2 mm pitch and read out by silicon photomultipliers. A wax-based opaque scintillator with a scattering length of approximately 0.5 mm is used. The tracking performance of this LiquidO detector is characterised with cosmic-ray muons and the position resolution is demonstrated to be 450 m per row of fibres. These results highlight the potential of LiquidO opaque scintillator detectors to achieve fine spatial resolution, enabling precise particle tracking and imaging.

Paper Structure

This paper contains 9 sections, 3 equations, 7 figures.

Table of Contents

  1. Introduction
  2. The 64-fibre Cube detector
  3. Data acquisition and grouping methods
  4. Offline muon selection
  5. Linearisation of the Time-over-Threshold signal
  6. Muon light cylinders
  7. Tracking and position resolution
  8. Comparison with state-of-the-art muon-imaging scintillator-based detectors
  9. Conclusion

Figures (7)

  • Figure 1: Images of the 64-fibre Cube detector. (a) A three-dimensional graphical representation of the detector displaying the internal components. (b) A photo of the setup, composed of the instrumented detector in between the two pixelated muon taggers, placed above and below the detector and enclosed in a black housing. (c) A photo of the detector not filled with scintillator, showing the fibre lattice. (d) A photo of the instrumented detector alone, with the readout ASICs and SiPMs attached and the lid on.
  • Figure 2: Photos of the interior of the 64-fibre Cube detector filled with the scintillators used: the liquid transparent mixture (left) and the opaque NoWaSH (right).
  • Figure 3: Probability map of ToT values as a function of the number of p.e. For each p.e. value, pulses replicating experimental signals are generated using a Monte-Carlo simulation, and used to calculate the corresponding ToT values and the ToT probability distribution. The logarithmic dependence of ToT on the number of p.e. is modelled with the fit function shown by the red line, which is used to estimate p.e. counts from raw experimental ToT values and correct for nonlinearity. The region between the dashed light blue lines represents the 1 $\upsigma$ uncertainty in the p.e.-to-ToT conversion, arising from uncertainties in the simulation parameters. The orange regions highlight the range of experimental ToT values obtained for muon events, showing that they predominantly fall within the region where ToT behaviour is closer to linearity. Specifically, the darker (lighter) orange region represents the range of ToT values that contribute 68% (90%) of the total p.e. in a muon event on average. The inset shows the 1 p.e. pulse measured from the TIA of PETsys ASIC and the fit used to characterise the average rise ($\uptau_r$) and decay ($\uptau_d$) times used in the simulation. The pulse shape is dominated by the response of the readout electronics---primarily the shaping circuitry of the TIA---while the intrinsic SiPM pulse shape has negligible impact on the overall signal.
  • Figure 4: Event displays of muons crossing the Cube. Each image represents the 8$\times$8 grid of WLS fibres and the colour scale shows the number of p.e. detected by each fibre. Events are grouped in pairs, selecting muons going through the same PixTags pixels in the transparent (left of pair) and opaque (right of pair) datasets. The size of the circles is exaggerated compared to the real diameter of the fibres for display purposes. The small purple rectangles above and below each image represent the Cube entry (top) and exit (bottom) location of the muon as estimated from the maximum signal in the top and bottom PixTags.
  • Figure 5: (a) Distribution of the number of SiPMs with signal and (b) the total number of photoelectrons in muon events. The histograms correspond to data taken with the detector filled with either opaque (orange) or transparent (grey) scintillators. A Gaussian function is used to fit the distribution of SiPMs with signal, while a Landau function is applied to the total number of p.e., as it is expected to describe the energy loss of cosmic ray muons. When using the opaque scintillator, more light is detected and, furthermore, these photons are distributed across fewer SiPMs, demonstrating light confinement.
  • ...and 2 more figures